Return the forge to GitHub: releases, self-update, links, CI

The GitHub account is restored, so github.com/ZacharyZhang-NY/Kigi-CLI is
the canonical repo again. Reverse the 2026-08 Gitea migration:

- .github/workflows/{release,warm-cache}.yml: restore the GitHub-hosted
  pipeline dropped in 3e1652c, unchanged — hosted runners, rust-cache,
  softprops release with every asset attached atomically.
- .gitea/workflows/release.yml: retire the Gitea edition (the runners are
  being deregistered; git.zacharyzhang.com keeps the v0.1.12/v0.1.13
  releases for old clients).
- kigi-env: update_base_url now defaults to the GitHub Releases API. The
  updater already speaks GitHub wire format and sends a User-Agent; v0.1.14
  is published to BOTH forges with identical assets so pre-0.1.14 installs
  polling Gitea self-update onto the GitHub channel.
- install.sh / install.ps1: resolve releases from api.github.com.
- feedback, /docs, SECURITY.md, frontend worker + site: repo, issues and
  raw-file links point at github.com / raw.githubusercontent.com.
- docs/RELEASE.md: GitHub edition of the checklist; Gitea runner sections
  retired.
- version: 0.1.13 -> 0.1.14 (the cross-forge transition release).

Claude-Session: https://claude.ai/code/session_01A6RcwHhhsFYYrNdQB4ASuP

crates/codegen/kigi-shell/src/agent/feedback_client.rs

@@ -13,7 +13,7 @@ use serde::Serialize;
 
 /// Where non-subscription users (no OAuth session) submit feedback instead.
 pub const FEEDBACK_ISSUES_URL: &str =
-    "https://git.zacharyzhang.com/ZacharyZhang-NY/Kigi-CLI/issues";
+    "https://github.com/ZacharyZhang-NY/Kigi-CLI/issues";
 
 /// HTTP error from the feedback endpoint with a preserved status code, so
 /// callers can distinguish auth failures (401) without string matching.